journald: set a lower size limit for FDs from unpriv processes

Unprivileged processes can send 768M in a FD-based message to journald,
which will be malloc'ed in one go, likely causing memory issues.
Set the limit for unprivileged users to 24M.

Allow coredumps as an exception, since we always allowed storing
up to the 768M max core files in the journal.

Reported on yeswehack.com as #YWH-PGM9780-48

docs/ENVIRONMENT.md

@@ -671,6 +671,10 @@ SYSTEMD_HOME_DEBUG_SUFFIX=foo \
   specified algorithm takes an effect immediately, you need to explicitly run
   `journalctl --rotate`.
 
+* `$SYSTEMD_JOURNAL_FD_SIZE_MAX` – Takes a size with the usual suffixes (K, M, ...) in
+  string format. Overrides the default maximum allowed size for a file-descriptor
+  based input record to be stored in the journal.
+
 * `$SYSTEMD_CATALOG` – path to the compiled catalog database file to use for
   `journalctl -x`, `journalctl --update-catalog`, `journalctl --list-catalog`
   and related calls.